Skip to main content
added 187 characters in body
Source Link
Iosif Pinelis
  • 127.7k
  • 8
  • 107
  • 229

As shown in my previous answer, the value of the sum that you see is $$\lim_{t\uparrow1}\sum_{n=2}^\infty (-t)^n \ln n. $$

Here is a more "manual" way to show this. Writing \begin{equation} \ln n=\int_0^\infty dz\,\frac{e^{-z}-e^{-nz}}z, \end{equation} for $t\uparrow1$ we have \begin{equation} \begin{aligned} &\sum_{n=2}^\infty (-t)^n \ln n \\ &=\sum_{n=2}^\infty (-t)^n \int_0^\infty dz\,\frac{e^{-z}-e^{-nz}}z \\ &=\int_0^\infty \frac{dz}z\sum_{n=2}^\infty (-t)^n (e^{-z}-e^{-nz}) \\ &=\frac{t^2}{1+t}\int_0^\infty \frac{dz}z\,e^{-z}\frac{1-e^{-z}}{1+t e^{-z}} \\ &\to\frac12\int_0^\infty \frac{dz}z\,e^{-z}\frac{1-e^{-z}}{1+e^{-z}} = \frac12\,\ln\frac\pi2. \end{aligned} \end{equation}\begin{equation} \begin{aligned} &\sum_{n=2}^\infty (-t)^n \ln n \\ &=\sum_{n=2}^\infty (-t)^n \int_0^\infty dz\,\frac{e^{-z}-e^{-nz}}z \\ &=\int_0^\infty \frac{dz}z\sum_{n=2}^\infty (-t)^n (e^{-z}-e^{-nz}) \\ &=\frac{t^2}{1+t}\int_0^\infty \frac{dz}z\,e^{-z}\frac{1-e^{-z}}{1+t e^{-z}} \\ &\to\frac12\int_0^\infty \frac{dz}z\,e^{-z}\frac{1-e^{-z}}{1+e^{-z}} \\ &=-\int_0^1 \frac{dx}{\ln x}\,\frac{1-x}{1+x} = \frac12\,\ln\frac\pi2, \end{aligned} \end{equation} by (say) Formula 4.267.1 on p. 545 in I.S. Gradshteyn and I.M. Ryzhik, Table of Integrals, Series, and Products, Seventh Edition, 2007.

As shown in my previous answer, the value of the sum that you see is $$\lim_{t\uparrow1}\sum_{n=2}^\infty (-t)^n \ln n. $$

Here is a more "manual" way to show this. Writing \begin{equation} \ln n=\int_0^\infty dz\,\frac{e^{-z}-e^{-nz}}z, \end{equation} for $t\uparrow1$ we have \begin{equation} \begin{aligned} &\sum_{n=2}^\infty (-t)^n \ln n \\ &=\sum_{n=2}^\infty (-t)^n \int_0^\infty dz\,\frac{e^{-z}-e^{-nz}}z \\ &=\int_0^\infty \frac{dz}z\sum_{n=2}^\infty (-t)^n (e^{-z}-e^{-nz}) \\ &=\frac{t^2}{1+t}\int_0^\infty \frac{dz}z\,e^{-z}\frac{1-e^{-z}}{1+t e^{-z}} \\ &\to\frac12\int_0^\infty \frac{dz}z\,e^{-z}\frac{1-e^{-z}}{1+e^{-z}} = \frac12\,\ln\frac\pi2. \end{aligned} \end{equation}

As shown in my previous answer, the value of the sum that you see is $$\lim_{t\uparrow1}\sum_{n=2}^\infty (-t)^n \ln n. $$

Here is a "manual" way to show this. Writing \begin{equation} \ln n=\int_0^\infty dz\,\frac{e^{-z}-e^{-nz}}z, \end{equation} for $t\uparrow1$ we have \begin{equation} \begin{aligned} &\sum_{n=2}^\infty (-t)^n \ln n \\ &=\sum_{n=2}^\infty (-t)^n \int_0^\infty dz\,\frac{e^{-z}-e^{-nz}}z \\ &=\int_0^\infty \frac{dz}z\sum_{n=2}^\infty (-t)^n (e^{-z}-e^{-nz}) \\ &=\frac{t^2}{1+t}\int_0^\infty \frac{dz}z\,e^{-z}\frac{1-e^{-z}}{1+t e^{-z}} \\ &\to\frac12\int_0^\infty \frac{dz}z\,e^{-z}\frac{1-e^{-z}}{1+e^{-z}} \\ &=-\int_0^1 \frac{dx}{\ln x}\,\frac{1-x}{1+x} = \frac12\,\ln\frac\pi2, \end{aligned} \end{equation} by (say) Formula 4.267.1 on p. 545 in I.S. Gradshteyn and I.M. Ryzhik, Table of Integrals, Series, and Products, Seventh Edition, 2007.

Source Link
Iosif Pinelis
  • 127.7k
  • 8
  • 107
  • 229

As shown in my previous answer, the value of the sum that you see is $$\lim_{t\uparrow1}\sum_{n=2}^\infty (-t)^n \ln n. $$

Here is a more "manual" way to show this. Writing \begin{equation} \ln n=\int_0^\infty dz\,\frac{e^{-z}-e^{-nz}}z, \end{equation} for $t\uparrow1$ we have \begin{equation} \begin{aligned} &\sum_{n=2}^\infty (-t)^n \ln n \\ &=\sum_{n=2}^\infty (-t)^n \int_0^\infty dz\,\frac{e^{-z}-e^{-nz}}z \\ &=\int_0^\infty \frac{dz}z\sum_{n=2}^\infty (-t)^n (e^{-z}-e^{-nz}) \\ &=\frac{t^2}{1+t}\int_0^\infty \frac{dz}z\,e^{-z}\frac{1-e^{-z}}{1+t e^{-z}} \\ &\to\frac12\int_0^\infty \frac{dz}z\,e^{-z}\frac{1-e^{-z}}{1+e^{-z}} = \frac12\,\ln\frac\pi2. \end{aligned} \end{equation}